As for organic light-emitting device, there was an example in the past to cause light emission by applying a voltage on an anthracene vapor-deposited film (Thin Solid Films, 94 (1982) 171) and the like. In recent years, however, in view of the advantages that a large area product can be obtained more easily as compared with an inorganic light-emitting device, desired color can be attained by development of various new materials and that it can be operated at low voltage and expected as a light-emitting device with a high-speed response and a high emission efficiency, application researches for device implementation as well as materials development are vigorously conducted.
For example, as described in detail in Macromol. Symp. 125, 1-48 (1997), an organic EL device has generally a structure comprising two layers of upper and lower electrodes formed on a transparent substrate and an organic layer comprising a light-emitting layer formed therebetween.
Recently, in addition to conventional devices utilizing fluorescence on transition from excited singlet state to ground state, devices utilizing phosphorescence via triplet exciton have been studied as represented by the following references: “Improved energy transfer in electrophosphorescent device” (D. F. O'Brien et al., Applied Physics Letters Vol. 74, No. 3, p. 422 (1999)) and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” (M. A. Baldo et al., Applied Physics Letters Vol. 75, No. 1, p. 4 (1999)). In these references, organic layers of four-layer structure are mainly used. They are composed of a hole-transporting layer, a light-emitting layer, an exciton diffusion-prevention layer and an electron-transporting layer from the side of an anode. The materials used are a carrier-transporting material and a phosphorescent material Ir(ppy)3.
Further, by using various types of fluorescent organic compounds, emission of a light of ultraviolet to infrared region, and, recently, researches on various compounds are actively conducted.
Moreover, other than the above-mentioned organic light-emitting devices using low molecule materials, an organic light-emitting device using a conjugated polymer was reported by a group of Cambridge University (Nature, 347, 539 (1990)). According to this report, a film of polyphenylenevinylene (PPV) was formed using a coating system and light emission from a single layer was confirmed.
As described above, the latest progress of organic light-emitting devices is remarkable, and their characteristic possibility of attaining a thin, lightweight, light-emitting device with a high luminance at a low applied voltage, a variety of emission wavelengths, and a high-speed response suggests their applicability to various uses.
However, at present, a light output of a higher luminance or a higher conversion efficiency is still required. Moreover, there are still many problems in respect of durability such as time-dependent change during prolonged use and degradation by an atmospheric gas including oxygen, moisture and the like. Furthermore, although in consideration of application to full-color display or the like, light emission of blue, green and red with a high color purity is needed, this problem has not been sufficiently resolved.
Moreover, although a number of aromatic compounds and condensed polycyclic aromatic compounds have been studied as a fluorescent organic compound for use in an electron-transporting layer and a light-emitting layer, etc., it is hard to say that those having sufficiently satisfying emission luminance and durability have been obtained.
Further, Japanese Patent No. 3,229,654 and Japanese Patent Application Laid-Open No. 2002-305084 can be mentioned as patent references to indole compounds related to those used in the present invention. However, they do not disclose those organic compound in accordance with the present invention characterized by having both a partial structure containing an indole ring and a partial structure containing a carbazole ring in a molecule structure.